Here's my initial "to get" list...
Classic Traveller (unsure)
Classic Adventures 1-13 [FFE reprint]
Classic Short Adventures (1-6+) [FFE reprint]
_________________________________________
MegaTravaller
MegaTraveller [Boxed Set]
(must have)
Accessories and Adventures
Astrogator's Guide to the Diaspora Sector
COACC: Close Orbit and Airspace Control Command
("got it")
Knightfall
(must have) 
Rebellion Sourcebook
Referee's Companion
("got it")
Digest Group Publications
World Builder's Handbook
_________________________________________
Traveller: The New Era
TNE Rulebook
(must have)
Accessories and Adventures
Fire, Fusion, & Steel: Technical Architecture
(must have)
The Regency Sourcebook: Keepers of the Flame
World Tamer's Handbook
_________________________________________
Mongoose Traveller
Adventure 1: Beltstrike
(unsure)
Adventure 2: Prison Planet
Supplement 2: Traders & Gunboats
Supplement 3: Fighting Ships
Traveller Compendium 1
The Third Imperium
Alien Module 1: Aslan
Alien Module 2: Vargr
Alien Module 3: Darrians
Reft Sector
Sector Fleet
The Spinward Marches
("got it")
Spinward Marches Map Pack
GURPS Traveller4
Interstellar Wars
Feel free to comment/suggest...